The present invention relates generally to coherent light sources that are wavelength tunable within a tuning bandwidth of the source and, more particularly, to such a tunable source with an optical gain element, such as a laser diode, and having an external cavity with an external light beam reflector which is simultaneously rotatable and translatable to provide for continuous linear wavelength tuning without external cavity mode hopping.
Semiconductor laser diodes typically operate in multiple longitudinal modes, i.e., at multiple frequencies. It is desirable, however, for these lasers for certain applications to operate in a single longitudinal mode over a tunable frequency range to provide single-frequency operation. Examples of such applications are disclosed in U.S. Pat. No. 5,392,308 to Welch et al., which is incorporated herein by reference thereto. Several configurations have been disclosed for arranging a diffractive grating along with or combined with other reflective elements and other optical elements together with a laser diode establishing an external optical path to insure single longitudinal mode tuning. Examples are shown in FIGS. 1A-1B. FIG. 1A illustrates a Littrow type configuration. In this configuration, laser diode 10 is combined with a rotatable reflective element grating 12, as indicated by arrow 14, via appropriate optics 16 to provide frequency selection feedback for laser diode 10. FIG. 1B illustrates a Littman type configuration. In this configuration, laser diode 10 is combined to form an external optical cavity with a fixed reflective element grating 12 and rotatable reflective element 18, as indicated by arrow 20, to provide frequency selection feedback for laser diode 10.
Many different kinds, variations and improvements have been suggested and disclosed based upon these two configurations, in particular, simplifying optical element alignment, manufacture and packaging of these external cavity tuned semiconductor laser diodes. An example is the Littman configuration shown in FIG. 2A involving a rotatable reflective element 18A comprising a prism, e.g., a right angle prism, of the type disclosed in U.S. patent application Ser. No. 08/497,435, filed Jun. 30, 1995 and entitled "Light Emitting Optical Device With External Retro-Reflector", assigned to the same assignee herein and incorporated by reference. The use of such a prism simplifies the external cavity alignment of the optical elements.
FIG. 2B is another illustration of the Littman configuration with simplified external cavity alignment wherein both reflective element grating 12 and reflective element 18 are rotatable as a unit, as indicated by arrow 22, as supported on a frame or platform 24.
Lastly, FIG. 2C illustrates a Littman platform configuration wherein the light source may be a flared semiconductor amplifier or may be a master oscillator power amplifier (MOPA) 10A. It will be apparent to those skilled in the art that there are many other possible combinations based upon either of the Littrow and Littman configurations.
For all of these different configurations, rotation of the movable element or elements, such as the reflective element or the reflective grating or a combination of both (hereinafter collectively referred to as "movable element"), will change the wavelength of the light propagating within the optical cavity. However, as changes are made in the laser wavelength, frequency changes will result in a discontinuous fashion as the oscillation frequency hops between distinct, spatial longitudinal modes of the optical cavity, unless special precautions are taken in the design of the movable element(s). It is a desirable feature, therefore, in such tunable external cavities that the longitudinal mode spectrum continually changes with frequency with the continuous rotation of the movable element(s) at the same rate of change in the preferred feedback wavelength resulting in truly continuous tuning of the output frequency of the external cavity. This condition is satisfied when the single-pass optical path length of the external cavity remains equal or nearly equal to the same integral number of half-wavelengths available across the tuning range of the laser cavity. It is a further desirable feature that the continuous tuning range of the laser cavity be as large as possible and that frequency-pulling effects resulting from slight deviations of the optical path length from the ideal are minimized.
The above mentioned features and conditions may be satisfied when the mechanical linkage that provides rotation of the movable element(s) also provides for a simultaneous translation of the element in such a way that the optical path length remains nearly constant as a function of wavelength change. Such simultaneous translation and rotation may be achieved by offsetting the center of rotation, i.e., the pivot point, of the movable element from the center of mass of the movable element. By proper choice of the location of the pivot point, the range of continuous tunability over the wavelength band of the laser diode may conceivably be maximized.
It is well known that, in order to avoid mode hopping between optical cavity longitudinal modes in an external cavity laser diode having an external grating mirror, the grating angle and the length of the external cavity must varied simultaneously so that the cavity longitudinal mode wavelength matches the grating wavelength. This is explained in U.S. Pat. No. 5,319,668 to Luecke as avoidance of tuning discontinuities by maintaining a constant integral number of half wavelengths in the external optical cavity over the entire wavelength tuning range available from the laser diode. U.S. Pat. No. 5,319,668 discloses a geometric construction for the location of the pivot point for a Littman configuration shown in FIG. 1B that employs a mirror as the movable element. The construction is carried out in such a way that the deviation of the optical path length from an integral number of half wavelengths, termed the "cavity phase error", is set equal to zero at three distinct wavelengths. This approach requires knowledge of the optical indices of refraction of all of the materials in the cavity at the three separate wavelengths. However, the construction disclosed in U.S. Pat. No. 5,319,668 does not describe or deal with external cavities based on a Littrow configuration, such as shown in FIG. 1A. Nor does the disclosure deal with the employment of a prism as a rotatable element or the combination of a grating and prism in fixed relation and rotatable as a unit, such as shown in FIGS. 2A and 2B. Furthermore, the method disclosed in U.S. Pat. No. 5,319,668 provides a residual cavity phase error that deviates from zero at wavelengths at positions between the three established wavelengths. This results in a small amount of frequency-pulling near the center of the wavelength band of the laser diode.
The geometric terms formulated by Luecke are indicated to take into account the effects of residual cavity phase error, including dispersion, as a function of wavelength caused by optical elements within the light path of the external cavity, in the calculation of mirror positions for three mirror different positions and corresponding wavelengths within the laser diode bandwidth. Such optical elements are the lenses, windows and air itself. Accordingly, the effectiveness of the three position wavelength calculation according to the methods disclosed in U.S. Pat. No. 5,319,668 is illustrated in FIG. 3 for comparison purposes with the instant invention illustrated in FIG. 4, which will be discussed in greater detail later.
It is an object of this invention to provide a external cavity, continuously tunable wavelength source, such as a laser diode device using an external cavity reflective grating, providing continuous wavelength tuning without longitudinal mode hopping.
It is a further object of this invention to provide a means to design a continuously tunable cavity with large tuning range based on the optical properties of the optical system at a single wavelength without any necessity for consideration of distinct plural wavelengths.
It is a further object of this invention to provide a design for the pivot point location of an external cavity laser in which the cavity phase error near the center of the wavelength band of the laser diode is made as small as possible.
It is another object of this invention to provide a design for the pivot point location of an external cavity laser that can be applied to a cavity configuration that include prisms, rotatable gratings, and rotatable grating/prism combinations that provide continuous single-frequency tuning over large wavelength bands.
It is another object of this invention to provide a continuously tunable laser diode device that provides only a single wavelength approach for reflective element error over the wavelength band of the laser diode with lateral deviation in cavity phase error remaining relatively small across the wavelength band of the laser diode.